The role of influenza vaccine in healthcare workers in the era of severe acute respiratory syndrome.

The new coronavirus responsible for severe acute respiratory syndrome (SARS) found its first victims in healthcare workers with attack rates of more than 50% before infection control measures were instituted;1,2 and this led to the near collapse of healthcare systems in some of the affected countries. In Singapore, healthcare workers accounted for 40% of the total number of SARS patients. The vulnerability of healthcare workers to this new respiratory disease should raise our awareness that this high-risk group may also be vulnerable to other well known respiratory diseases. One of these is influenza, but unlike SARS, influenza is vaccine preventable. The rapid dissemination of sudden acute respiratory syndrome around the world should be considered a rehearsal for the next pandemic of influenza.3 Even without an influenza pandemic, more people worldwide have died of complications of influenza during the SARS crisis than of SARS. Influenza and SARS have a lot in common. Considerable morbidity, social disruption and economic loss occurred during past influenza pandemics and the very recent SARS epidemic. Transmission is by droplet or direct contact. The majority of infected persons have symptoms of disease – which, in turn, enhance the likelihood of contagion.4 Mortality increases with age in both diseases. Both have non-specific early manifestations, with the most common early systemic symptoms being fever, malaise, myalgia and headache.3 Because of the similar presentations, misdiagnosis is likely and this was mirrored in the Singapore experience: in the beginning of the outbreak (before even the term SARS was coined) the index patients admitted at Tan Tock Seng Hospital (which later became the designated SARS hospital) were initially thought to suffer from avian influenza. Vice versa, at the tail end of the epidemic, alarms were raised when a new cluster of healthcare workers from the Institute of Mental Health developed fever. Fears that this could be a resurgence of SARS led to major disruptions within the institute and quarantining of healthcare workers until influenza B was shown to be the causative agent. Influenza and SARS also have a lot not in common. In addition to droplet and contact transmission, influenza is also associated with airborne transmission,4 whereas all current evidence points towards the absence of airborne transmission for SARS. Therefore, the SARS epidemic does not appear to have the spreading potential which characterises an influenza pandemic, during which millions of people can be affected within months around the world.5,6 The explosive nature of influenza outbreaks in groups aboard airplanes or cruise ships4 has also not been observed in association with SARS.7 The extent of infectiousness of influenza depends on the pre-existing immunity in the population, with high levels of immunity conveying protection, unless there was an antigenic shift or drift in the makeup of the virus. The SARS coronavirus hit a non-immune population, but at present it is unclear whether acquired antibodies to SARS (as demonstrated by a positive SARS serology) will lead to protection in the future or may be associated with a worse prognosis due to immune enhancement phenomena after repeated exposure. Unlike SARS, early laboratory diagnostic tests, an effective vaccine, proven antiviral therapy and chemoprophylaxis are available for influenza. Disease manifestation in influenza ranges from asymptomatic to severe disease and death with the majority having mild disease, whereas currently available data for SARS indicate that the vast majority of infected patients develop progressive pulmonary changes and around 15% die. The true range of clinical spectrum in SARS, however, remains to be established. Experts predict that another influenza pandemic is highly likely, if not inevitable.8,9 It is almost 35 years since the last pandemic, and the longest inter-pandemic interval recorded with certainty is 39 years.10 More than 20 million people died during the pandemic ‘flu season 1918-1919, the largest influenza pandemic of the 20th century. Whilst mortality on the scale